CLADDING TECHNOLOGY SHANXI CO., LTDExperimental Study on Magnetic Control Technology for Strip Electrode Overlay Current
Literature Overview and Technical Background
This 2005 publication from Xinjiang Agricultural University investigates the application of magnetic field control technology to strip electrode overlay welding (also known as twin-wire submerged arc welding or strip cladding). Strip electrode overlay welding is a highly productive process for depositing thick overlay layers on large components such as pressure vessel shells, heat exchanger tubesheets, and structural steel plates. However, conventional strip electrode welding suffers from arc instability, irregular bead profile, and difficulty in controlling the molten pool geometry—issues that are exacerbated by the wide, flat geometry of the strip electrode compared to conventional round wire.
The introduction of an external magnetic field to control the strip electrode welding arc represents a novel approach to improving process stability and deposit quality. The magnetic field interacts with the electric current flowing through the arc and the molten pool, generating Lorentz forces that influence arc shape, molten pool convection, and solidification pattern.
Core Technical Principles
Magnetic Field Configuration
The magnetic field can be applied in several configurations, each producing different effects on the welding process:
| Magnetic Field Type | Direction | Primary Effect |
|---|---|---|
| Longitudinal | Along welding direction | Arc elongation, increased penetration |
| Transverse | Perpendicular to welding direction | Arc deflection, bead profile control |
| Vertical | Perpendicular to workpiece surface | Molten pool stirring, grain refinement |
| Rotating | Rotating around arc axis | Dynamic stirring, uniform heat distribution |
| Oscillating | Alternating direction | Arc oscillation, wider bead |
Physical Mechanism
The Lorentz force generated by the interaction of the magnetic field (B) and the welding current (I) is given by:
F = J × B × L
where J is the current density, B is the magnetic flux density, and L is the characteristic length of the arc or molten pool. This force acts on the charged particles in the arc plasma and on the currents induced in the molten pool, producing:
- Arc stabilization: The magnetic field constrains the arc to a more predictable shape, reducing arc wandering and improving weld consistency.
- Molten pool stirring: The Lorentz force drives convection currents in the molten pool, promoting more uniform temperature and composition distribution.
- Solidification control: The enhanced convection promotes columnar-to-equiaxed transition, resulting in finer grain structure and reduced segregation.
- Bead profile control: By controlling the direction and magnitude of the magnetic field, the weld bead width, reinforcement, and profile can be precisely controlled.
Experimental Parameters and Results
Process Parameters
| Parameter | Range | Typical Value |
|---|---|---|
| Strip electrode width | 20–40 mm | 30 mm |
| Strip electrode thickness | 0.5–1.5 mm | 1.0 mm |
| Welding current | 1000–2500 A | 1800 A |
| Arc voltage | 25–35 V | 30 V |
| Travel speed | 300–600 mm/min | 450 mm/min |
| Magnetic field strength | 0.1–1.0 T | 0.5 T |
| Flux type | Basic or rutile | Basic |
| Pre-heat | 100–200 °C | 150 °C |
| Shielding gas | None (SAW) | — |
Performance Comparison
| Parameter | Conventional Strip Electrode | Magnetic Field Controlled | Improvement |
|---|---|---|---|
| Arc stability | Moderate | Excellent | Significantly improved |
| Bead width variation | ±15–20% | ±5–8% | 50–60% reduction |
| Bead reinforcement variation | ±15–25% | ±5–10% | 50–60% reduction |
| Grain size | 150–300 μm | 80–180 μm | 40–50% refinement |
| Deposition rate | 8–12 kg/h | 8–12 kg/h | Maintained |
| Dilution rate | 10–18% | 8–14% | 15–25% reduction |
| Hardness uniformity | ±30 HV | ±15 HV | 50% improvement |
Engineering Applications
The magnetic field control technology for strip electrode overlay welding has several important engineering applications:
Pressure Vessel Cladding
For large-diameter pressure vessels requiring thick overlay layers (e.g., hydrogenation reactors with 10–20 mm Ni-based alloy cladding), strip electrode welding is the preferred process due to its high deposition rate. Magnetic field control improves the consistency of the overlay layer, reducing the risk of thin spots or excessive dilution that could compromise the corrosion resistance of the cladding.
Heat Exchanger Tubesheet Cladding
Heat exchanger tubesheets often require a corrosion-resistant overlay (e.g., 316L SS or Monel 400) on a carbon steel or low-alloy steel base. The strip electrode process with magnetic field control produces a uniform overlay thickness across the tubesheet, which is critical for ensuring consistent corrosion protection around tube holes.
Structural Steel Surface Protection
In marine and offshore applications, structural steel components require corrosion-resistant overlay layers. The magnetic field controlled strip electrode process provides a cost-effective means of depositing thick, uniform overlay layers on large flat or curved surfaces.
Defect Analysis and Countermeasures
| Defect | Cause | Magnetic Field Effect |
|---|---|---|
| Arc wandering | Uneven flux distribution, geometry variation | Magnetic field constrains arc, reducing wandering |
| Bead profile irregularity | Current distribution variation | Magnetic field stabilizes current distribution |
| Porosity | Gas entrapment from flux | Improved arc stability reduces gas entrapment |
| Cracking | High residual stress | Enhanced convection reduces stress concentration |
| Dilution variation | Inconsistent penetration | Magnetic field controls penetration depth |
Study Insights and Conclusions
This 2005 research represents an innovative approach to improving the quality and consistency of strip electrode overlay welding through magnetic field control. The fundamental insight—that the Lorentz force generated by the interaction of magnetic field and welding current can be harnessed to stabilize the arc and improve molten pool dynamics—is both physically sound and practically valuable. The experimental results demonstrate that magnetic field control can significantly reduce bead profile variation, refine grain structure, and improve hardness uniformity while maintaining the high deposition rate that makes strip electrode welding attractive for thick overlay applications. For engineers working on large-scale cladding operations such as pressure vessel fabrication and heat exchanger manufacturing, this technology offers a promising means of improving process capability without sacrificing productivity. The research underscores the broader principle that electromagnetic control of welding processes is a powerful tool for achieving precise metallurgical and geometric outcomes in high-deposition-rate overlay applications.